A Twitter card checker, often called an X card validator, reads the meta tags that control how a link looks when it is posted on X. Enter a URL and this tool fetches the page, shows a preview of the card and checks the four core tags: twitter:card, twitter:title, twitter:description and twitter:image.
Each tag is marked present or missing, with its value. The preview uses the Twitter tags first and falls back to og:title and og:description when they are missing, which is also what X does. twitter:card sets the layout: summary shows a small square image beside the title, and summary_large_image shows a wide image above it.
Limits: the preview approximates X's layout. X caches cards, crops images its own way, and may not show a card if the image is blocked or too large. The tool reads the HTML before JavaScript runs, so tags added by JavaScript are not seen, and X's crawler doesn't run JavaScript either.